Despite your reboots, which I do think are an effort in the right direction, I can see two potentials here. One is local, with some sort of duplicate IP address issue. You need to ensure the Sonos devices are unplugged from power when you reboot the router, otherwise they’ll store the IP address and try to reconnect to the same one when the router comes up, and depending on which IP is ‘duplicated’ it’s possible it won’t clear the issue properly.

The other, of course, is the possibility of a issue with the Spotify server itself, or, I suppose, the path outside your local network to the Spotify servers. I see this as a fairly low possibility. 
 

I’d try the reboot sequence as described in my first paragraph. In the unlikely occurrence it doesn’t resolve the issue, I’d cause the problem to happen again, and then submit a system diagnostic and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

The fact the two speakers are wired, leads me to first suggest to maybe try them on your WiFi signal instead, or perhaps try a new cable for each speaker - I’m assuming both are wired direct to the router, but if they are wired to a separate switch, then maybe try a different switch too.

This link may assist you, if you decide to put your speakers to run on the local WiFi…

https://support.sonos.com/s/article/3209

This is interesting. I have the same issue.  All my sonos play devices are wired to the router and we only joined spotify recently but straight off, I noticed songs (every third or fourth) skipping just as mentioned here and in many other threads, some going back several years.

Diagnostic 270854728

What did Sonos support say when you called in with that diagnostic number?

Hi Airgetlam

No direct response however I did just have to update my desktop controller - now was this a general update or one sent to my system specifically (kudos if so). Anyway, since that update I have not had the problem…

I am on System OS1 / Version 11.6.1

Magic happened and the problem fixed itself!!